1. Prior Art
The prior art is believed to be best exemplified by the following patents and literature references:
Hochwalt et al, U.S. Pat. No. 2,168,160, July 1939, and Hochwalt et al., U.S. Pat. No. 2,282,518, May, 1942, relate to production of phenol-aldehyde-lignin resins that are useful in molding compounds and in which lignin containing about 80% by weight water or a desulfonated lignosulfonate is dissolved in phenol, admixed with aldehyde and heated to effect condensation in the presence of either an acid or alkaline catalyst. The references appear to involve condensations of the resole type.
Ball, U.S. Pat. No. 3,185,654, May, 1965, relates to reaction of lignin with phenol-formaldehyde A-stage resin to produce resole type resins.
Salimsakov et al., Plast, Massy, 12, 61-3 (1973) [Chemical Abstracts, 78, 112897a, 1973], relates to the use of hydrolysis lignin as a filter for phenol-formaldehyde resins in the production of molding powders.
Doi et al., Japan No. 70/20,308, July (1970) [Chemical Abstracts 74, 64926s (1971)], relates to rapid curing phenolic varnishes produced by reaction of phenol, formaldehyde, toluenesulfonamide and lignin in the presence of a basic catalyst followed by incorporation of hexamethylenetetramine in the resin thus obtained.
Kalnins et al., Akad. Nauk. Lalv., SSR, 24, 65-70 (1962) [Chemical Abstracts, 58, 5880b (1963)], prepared molding powders by precipitation of lignin from spent hydrotropic liquor and reaction of the lignin with phenol to form a condensate followed by reaction of the condensate with formaldehyde to form novolak resins.
Apel et al., U.S. Pat. No. 2,956,033, November, 1960, relates to phenol-aldehyde-lignin resins of the novolak type suitable for molding prepared from "low temperature" lignin obtained as a waste product in a saccharification process. A phenol-formaldehyde condensate is first produced which is then reacted with phenol, sulfuric acid and hydrolysis lignin. The particular hydrolysis lignin is disclosed to be critical and other hydrolysis lignins are said to be unsuitable in the process.
Clark, U.S. Pat. No. 2,520,913, September/1950, relates to production of a resin from reaction of cresylic acid, phenol and formaldehyde and, optionally, hydrolysis lignin, obtained by the saccharification of corn cobs, in the presence of sulfuric acid. The resultant resin is neutralized, washed with water, and compounded with hydrolysis lignin and hexamethylenetetramine to produce a molding material.
Donahue et al., Plastics, 1, No. 2,45,101 (1944) discloses hydrolysis lignin as a filler in phenol-formaldehyde molding compounds.
Mori et al., Hokkaidoritsu Rinsan Shikenjo Kenkyu Hokoku 1969, No. 53, 45-81 (Japan) [Chemical Abstracts 74, 127848r, (1971)] relates to production of foamable lignin resins by resole type condensation of alkali lignin with formaldehyde and phenol.
Popova et al., Tezisy Dokl.--Vses. Konf. Khim. Ispol'z Lignina, 6th, 1975 (Pub. 1976), 160-3 (Russia) [Chemical Abstracts 86, 173343t (1977)], discloses compression molding compositions derived from a powdered composition of sawdust, phenol, formaldehyde and HCl.
Grigor'ev et al., Tr. Leningrad. Tecknol. Inst. Tsellyul.--Bum. Prom. No. 21, 199 (1968) [Chemical Abstracts, 71, 72120c (1969)], relates to studies of condensation products of lignin and ammonium lignosulfonate with phenol and formaldehyde.
2. Field of the Invention
This invention relates to ligno-novolak resin blends and synthetic ligno-novolak resins; to molding compositions derived from such resins and resin blends; and to cured, crosslinked derivatives of such resins and resin blends.
Lignin is derived from wood as a by-product in the pulping process and, as an abundant, natural and renewable product, it has tremendous potential for many industrial uses as a replacement for increasingly scarce and expensive petroleum based materials. Lignin has found limited utilization commerically, however, because it is complex chemically and physically and its characteristics have been found to vary considerably, depending on many factors including variation in pulping conditions, the kind of wood being pulped, the conditions under which it is recovered, etc.
It is known that lignin obtained as a by-product in alkaline pulping processes contains both aliphatic and aromatic hydroxyl groups that are attractive sites for chemical modifications. Indeed, because of the well known guaiacyl moiety present in lignin and its derivatives, lignin has been proposed as a replacement for phenol in the production of phenolic resins. In view of the ever-rising cost of phenols and potential shortages due to diminishing resources, the use of such lignin substitutes from renewable and natural sources becomes increasingly more attractie.
One technological area in which lignin has been proposed as a phenol substitute has been in the production of phenol-formaldehyde type resins. These resins have generally been categorized as "resoles", i.e., resins produced from reaction of a molar excess of formaldehyde with phenol, normally in the presence of alkaline catalysts and curable without the necessity of a curing agent, or "novolaks", i.e. resins produced from reaction of a molar excess of phenol with formaldehyde generally in the presence of an acid catalyst, and curable in the presence of a substantial quantity of a curing agent such as hexamethylenetetramine.
Where lignin has been proposed in the production of novolak resins for molding compounds, it has in general been viewed as a filler or it has been employed as a reactant, replacing a portion of the phenol being reacted with formaldehyde. Many of such prior proposals employ hydrolysis lignin for such purposes. Such hydrolysis lignin is a highly purified material derived from acid hydrolyzates which are in turn derived by digestion of lignocellulosic materials with dilute aqueous acid at temperatures ranging from about 190.degree. to 225.degree. C. and pressures of about 200 to 400 psi, normally under the action of injected steam. The acid hydrolyzates will contain sugars such as xylose or glucose, furfural, humins, lignin decomposition products such as vanillin or other aromatic compounds. Hydrolysis lignin is the solid left in the hydrolyzate after separation of the sugars and other components. In other words, hydrolysis lignin is a highly purified, chemically altered lignin material. Alkali or Kraft lignin, on the other hand, is obtained as a by-product of alkaline pulping using either the soda process in which the pulping liquor contains sodium hydroxide or the sulfate process, wherein the pulping liquor contains both sodium hydroxide and sodium sulfide. During the pulping process, the lignin becomes dissolved in the pulping liquor as a salt of lignin and is conventionally recovered by acid precipitation as either free lignin or as a lignin salt depending upon the specific conditions under which the lignin is obtained. The two types of lignin materials, i.e., alkali lignin and hydrolysis lignin, are quite different as is well recognized in the art.
Heretofore, whether the phenol replacement has been proposed in the synthesis of resole or novolak resins employing hydrolysis lignin or otherwise, there has been little or no commercial use of lignin in the production of such resins. This has primarily been the result of processing difficulties that have been encountered, such as slow cure, high water absorptivity, plating out in the molding process, etc., as well as adverse effects upon the properties as compared with the properties of the resins produced without phenol replacement.
It would, of course, be desirable to optimize and, indeed, improve the properties of novolak type resinous products while also realizing the cost savings that are possible through utilization of a less expensive substitute. Moreover, it would be most advantageous to achieve such optimization and savings in a manner that is versatile, convenient and straightforward, and that requires no special or expensive equipment or procedures to carry out.
Accordingly, a primary object of the present invention is to provide a novel formulation for producing a thermoset lignin/phenol/aldehyde resin of the novolak type that exhibits desirable properties and in which lignin is interreacted and crosslinked.
Another objective of the present invention is to provide novel lignin/phenol/aldehyde resins suitable for use in the production of molding compounds.
Another object of the invention is to provide novel novolak molding compounds comprised of such ligno-novolak resins, which compounds exhibit properties that are at least equivalent, and in some respects superior, to those of conventional novolak molding compounds.
It is also an object of the invention to provide such formulations and molding compounds which exhibit desirable processing and molding characteristics.
Yet another object of the invention is to provide such formulations and molding compounds which can be produced in a manner that is both versatile and convenient and also relatively inexpensive.